Wikimedia projects have certain founding principles in common. These principles may evolve or be refined over time, but they are considered ideals essential to the founding of the Wikimedia projects – not to be confused with the Wikimedia Foundation (which also arose from the Wikimedia projects). People who strongly disagree with them are nonetheless expected to either respect them while collaborating on the site or turn to another site. Those unable or unwilling sometimes end up leaving the project.

Maintaining room for fiat to help resolve particularly difficult problems. On a dozen projects, an Arbitration Committee has the authority to make certain binding, final decisions such as banning an editor.

Some do not rely on NPOV (Commons, which says "Commons is not Wikipedia, and files uploaded here do not necessarily need to comply with the Neutral point of view"), or have a simpler principle of 'being fair' (Wikivoyage, which says "Travel guides should not be written from a neutral point of view").

Some have primarily non-wiki modes of collaboration and decision-making (MediaWiki).